1. Field of the Invention
The present invention relates to a burst mode optical receiver using two amplifiers having different bandwidth, and in particular to an improved burst mode optical receiver using two amplifiers having different bandwidth which is capable of significantly improving a receiver sensitivity without using a preamble bit.
2. Description of the Conventional Art
Recently, the TDMA(Time Division Multiple Access) using a fast packet signal for a high speed multimedia signal transmission has been intensively investigated. In a central office of the above-described system, in order to reduce subscriber cost, the packet signals from various subscribers are received using one optical receiver. Therefore, the size and phase of the received packet signals are different for every packet.
These signals are called a burst signal. The burst mode optical receiver is used for receiving these burst signals.
In a conventional poing-to-point optical communication system, the output of the linear channel is AC coupled to the decision circuit to fix decision threshold voltage as a constant. In order to receive the burst data using the optical receiver, an idle time(sum of guard a time and a preamble time) should be increased between the packets. However, if the idle time is increased, the transmission efficiency of the packet is decreased. If the capacity of the coupling condenser is decreased for decreasing the idle time, another apparatus for coding/decoding the data which are transmitted is needed. Therefore, a burst mode optical receiver is disclosed, which is capable of coping with various size variations of an input signal with a short idle time and having a wide dynamic range.
In the burst mode optical receiver, in order to remove the effects of the coupling condenser, a DC-coupling method is used differently from the conventional method. In addition, the power variation of the input signal is detected and is fed back to a pre-amplifier or to an amplifier of the next circuit for thereby obtaing the optimum threshold value of the decision circuit (in most case, a limiting amplifier) based on the power variation of the input signal. The burst mode optical receiver is classified into a feed back type and a feed forward type based on the compensation type.
The burst mode receiver of the feed back type disclosed in U.S. Pat. No. 5,025,456 (Ota and Swartz) is formed of a pre-amplifier and a limiting amplifier having a differential input. The variation in the average value of the input signal is detected from an output signal of the pre-amplifier using the optimum detection circuit and is fed back to a differential input of the input terminal of the pre-amplifier for thereby receiving a quickly changing burst signal. In addition, in order to enhance the performance of the receiver, a method for decreasing an extinction ratio penalty and a method for receiving a low and high speed data are disclosed in the article of the 1 Gb/s burst mode optical receiver. In this method, the pre-amplifier and limiting amplifier are designed to have a differential input and output. This burst mode optical receiver has a 1.5 dB penalty compared to the conventional optical receiver in view of the receiver sensitivity. If the signal is not inputted, a proper off-set voltage should be set in order to prevent an increase of error.
As another method for implementing the burst mode optical receiver, there is a feed-forward method disclosed by Nakamura, et al., xe2x80x9cAn Instantaneous Response CMOS Optical Receiver IC with Wide Dynamic Range and Extremely High Sensitivity Using Feed-Forward Auto-Bias Adjustmentxe2x80x9d, Journal of Solid-State Circuits, Vol. 30, No. 9, September 1995, pp. 991-997. In this method, a maximum value and minimum value of the signal is detected at the output of the preamplifier and a mean value is computed. This mean value is a threshold value for determining a level 1 and a level 0 of the signal by the limiting amplifier.
In this method, this method is easily implemented. Since the threshold voltage is extracted from a noise-contained 1bit sampling voltage for thereby causing a 1.5 dB power penalty same as the previously described method.
In addition, in another method, a preamble bit is inserted irrespective of the data to be transmitted to the front of the packet for decreasing the 1.5 dB power penalty compared to the conventional burst mode optical receiver. In the receiving terminal, the received preamble bits are averaged for thereby controlling a threshold voltage and decreasing the noise of the threshold voltage control signal and the penalty.
Accordingly, it is an object of the present invention to provide a burst mode optical receiver using two amplifiers having different bandwidth which overcomes the aforementioned problems encountered in the conventional art.
It is another object of the present invention to provide a burst mode optical receiver using two amplifiers having different bandwidth which is capable of improving a receiver sensitivity without using a preamble bit.
In order to achieve the above objects, there is provided a burst mode optical receiver using two amplifiers having different bandwidth which includes an optical coupler for splitting an input optical signal into two parts and outputting the split optical signals, a delay circuit formed of a first optical fiber delaying an optical signal packet outputted from the optical coupler by a half of packet duration and a second optical fiber passing the optical signal packet without delaying the optical signal packet, a first photodiode for receiving a first optical signal on the first optical filter and for generating a first electrical signal from the received first optical signal, a second photodiode for receiving a second optical signal on the second optical fiber and for generating a second electrical signal from the received second optical signal, a first pre-amplifier having a wide bandwidth for receiving a first electrical signal from the first photodiode and amplifying the first electrical signal, and a second pre-amplifier having a narrow bandwidth for receiving a second electrical signal from the second photodiode and amplifying the second electrical signal.
In the present invention, a novel burst mode optical receiver capable of decreasing a 1.5 dB penalty is disclosed. The optical receiver is formed of an optical coupler, an optical delay unit, and two pre-amplifiers having different bandwidth, so that it is possible to extract a threshold voltage control signal by integrating data and removing noise without using the preamble bits.
Additional advantages, objects and other features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly leveled out in the appended claims as a result of the experiment compared to the conventional arts.